Multiplex carrier wave transmission



April 18, 1939. M. A. WEAVER MULTIPLEX CARRIER WAVE TRANSMISSION 1 M tOn N w R M 0 T m NE A MW ww WA 2 M V, B m 9 l 9 p e s d m i F April 18,1939. M. A. WEAVER MULTIPLEX CARRIER WAVE TRANSMISSION Filed Sept. 9,19s? 2 Sheets-Sheet 2 ATTORNEY Patented Apr. 18, 1939 UNITED srATasPATENT orrica MULTIPLEX CARRIER WAVE TRANSMISSION Application September9.19:1. Serial No. 103,111

This invention relates to carrier wave communication systems, and moreparticularly to multiplex transmission systems and to the reduction ofthe effects thereon of atmospheric disturbances, cross-talk, noise orthe like.

Most, if not all, carrier communication systems are aifeeted in somedegree by electrical disturbances that arise in the system and tend tointerfere with the intelligence bearing waves to be transmitted. Innearly all cases the disturbing effects are not of the same intensitythroughout the operating frequency range but are a function offrequency. Thus, for specific example, in open wire carrier systemsstatic and cross-talk from adjacent carrier systems tend to increasewith frequency; in shielded transmission systems, such for example, as acoaxial conductor system, the noise due to atmospheric disturbancesdecreases with frequency; and in 2o cables where the circuits areseparated by an effective shield, cross-talk between circuits tends todecrease with increase in frequency. Where signals are transmitted overany such system in a multiplicity of respective frequency bands,

some signals'are therefore subjected to greater interference thanothers. Some carrier circuits, accordingly, may be excessively noisywhereas others may be unnecessarily quiet; but it is the condition ofthe noisiest circuitthat is the limiting factor in the design of thesystem.

In accordance with the present invention, each signal is not transmittedthroughout the length of the multiplex system in a respective frequencyband or channel, but it is systematically assigned to successivelydifferent channels as it progresses through the system. The system ofsuccessive assignments is such that the intensity of disturbing noise,cross-talk or the like is substantially the same for all circuits; andsubstantially less than it would be in the noisiest circuit if thesignals were confined to respective frequency channels.

The nature of the present invention, its advantages and its featureswill more fully appear in the following description of specific systemsembodying the invention, reference being made to the accompanyingdrawings in which:

Figures 1 and 2 show schematically a multiembodying the invention; and

Fig. 3 is a graph showing a typical disturbance-frequencycharacteristic.

Referring now more particularly to Fig. 1, the specific illustrativeembodiment of the invention therein depicted comprises a twelve-channelplex carrier telephone wire transmission system multiplex carrier wavetelephone system suitable for wire line transmission from a terminalstation W to another terminal station E, perhaps four thousand milesdistant. At several points along the system are intermediate stations A,B' and O which are presumably located in large cities, and connectingthe several stations are the transmission lines L1, 14, lo and L4 eachroughly one thousand miles'in length. These lines may be of any formsuitable for carrier wave telephone signals, such as an open wire line,a coaxial pair, or an ordinary shielded or non-shielded pair in amulti-pair cable, but the last-mentioned form will be assumed forpurposes of further discussion.

At terminal station W, modulating means M and band filters F1, F2, F3,etc.. are provided for impressing twelve difi'erent signals, speechsignals in this case, on respective carrier waves so that the resultingbands of signal-modulated waves occupy respective, adjacent positions inthe frequency spectrum ranging for specific example from 12 kilocyclesto 60 kilocycles. These waves are then applied to the transmission lineL1 for transmission thereover to the first intermediate station A'.

Each signal is designated by a letter A, B, C, D, etc. and the carrierwave on which it is impressed may be designated by its frequency f1, f2,fa, etc. The latter designations may be understood to apply also to thecarrier frequency channels through which the signal-modulated waves aretransmitted, f1 representing the channel of lowest frequency.

At the first intermediate station A, the various signal bands arrivingover the line L1 are separated by band filters F1, F2, F3, etc. and thesignals are reduced to their original voice frequency by demodulatingmeans DM and voice frequency filters F which are now well-known in theart. These various signals are then made to appear at respective outputterminals 0 at the intermediate station. On another panel at the samestation are the'input terminals 2 which may be cross-connected to any ofthe output terminals 0. Each input terminal is associated with meanssimilar to those at the terminal station W for impressing applied voicefrequency waves on a respective carrier wave, so that again twelvebands. of signal-modulated waves may be obtained and applied to thetransmission line L2.

The voice frequency patching or cross-connecting bay comprising the twoaforementioned panels permits the connection of local subscriberscircuits into the transcontinental car rier system, but this-feature isnot so important with respect to the objects of the present invention asthe mannerin which through calls are passed from one panel to the other.More specifically, in accordance with an embodiment of the presentinvention, signal A arriving over carrier channel I; of lowest frequencyis patched to carrier channel In of highest frequency and the othersignals are similarly patched to effect a frequency inversion of the12-60 kilocycle band.

At the next intermediate station B it will be found that there is not asgreat disparity in the relative cross-talk levels in the severalchannels as there was at station A, for no one signal has occupied acomparatively noisy channel for more than half the distance from theterminal station W. Repeated frequency inversion at successiveintermediate stations along the system might be effective in retainingsubstantially the same relative cross-talk level as obtained at stationB but in the usual case it would be found that some of the signals werefar more affected by cross-talk than others. This follows because therelation between cross-talk and frequency is in practice not a linearone. A typical crosstalk-frequency characteristic is shown in Fig. 3,the data for this characteristic curve being obtained by measurement ona system of the kind herein described in which cross-talk balancingdevices were applied at intervals along the cable. A closer approach tothe ideal condition of equal cross-talk in all of the carrier channelsis realized by following the system of successive channel assignments,following the initial frequency inversion, that is now to be described.

At station B, as will appear from Fig. l, signal A is assigned tocarrier channel 1, signal B to channel f4, C to fa, D to is, E to h, Fto Is, G to f12, H to I10, I to In, J (30 ft, K fs and L to f5. At thelast intermediate station C the process is repeated with each signalassigned to a carrier channel not previously traversed by that signal.Thus signal A is by frequency translation assigned to carrier channel[5, signal B to channel It, signal C to f6, signal D to In, E to in, Fto ho, G to f1. H to fa, I to 1'2, J to fa, K to 14, L to f1.

After transmission over the last section of line L4, the signals arriveat the terminal station E where the signals are translated to theiroriginal voice frequencies for distribution through the local telephoneexchange.

Fig. 2, which is based on Fig. 1, shows the successive carrier frequencyassignments of four representative signals A, D, J and L as they aretransmitted from one end of the system to the other.

To obtain a quantitative idea as to the extent to which cross- -talkdisparity is reduced by employing the scheme illustrated in Fig. 1,assume that Fig. 3 is applicable and that the cross-talk per thousandmile length .varies as there indicated from a value of unity to a valueof 2.5. Assume further that the phase of the cross-talk is random in thesuccessive intervals of the line. Under these circumstances, thecross-talk in the several through circuits is very nearly the same, thecomparable figures being 3.36 for signals A, E, G and L, 3.15 forsignals C, F, H and J, and 3.46 for the other four signals. Calculationwill show that in this case thecross-talk in the noisiest channels isless than five per cent greater than the average for all twelvecircuits.

A more nearly perfect averaging of the crosstalk in the carrier channelscan be obtained by increasing the number of links or intermediatestations in the system and properly applying the 1 principle ofsuccessive channel assignments. Thus, if the four thousand mile systemshown in Fig. 1'is provided with eleven intermediate stations instead ofthree, thus dividing the system into twelve transmission links eachabout 333 miles in length, the following is the preferred scheme ofchannel assignment. signal A will be transmitted over the first link ofthe system in carrier channel I1, over the second link in channel f2,over the third in h, and so on to the last link where the signaloccupies channel In. Signal B similarly starts in channel f2, progressesto channel In in the eleventh link and to channel ii in the last. Thefollowing schedule shows the channel assignment for each of the twelvesignals in each of the twelve links of the system.

Transmitted over link N 0.

Signal 1 2 3 4 5 6 7 8 9 10 ll 12 in carrier channel f-No.

A 1 2 3 4 5 6 7 8 9 10 ll 12 B 2 3 4 5 6 7 8 9 10 ll 12 l 3 4 5 6 7 8 910 ll 12 l 2 l) 4 5 6 7 8 9 l0 11 12 1 2 3 6 7 8 9 10 ll 12 l 2 3 4 7 89 10 11 l2 1 2 3 4 5 8 9 10 ll 12 l 2 3 4 5 6 9 10 ll 12 l 2 3 4 5 6 710 ll 12 1 2 3 4 5 6 7 8 ll 12 l 2 3 4 5 6 7 8 9 12 l 2 3 4 5 6 7 8 9 l01 2 3 4 5 6 7 8 9 10 ll In the foregoing examples of the presentinvention cross-talk has been assumed to be the source of interference,but this is for illustrative purposes only and other types ofinterference might have been assumed. If several types of interferenceare present in the system the intensityfrequency characteristic of eachmay be determined and a composite interference-frequency characteristicused as the basis of the scheme of successive channel aslgnments. Ifdesirable, the relative interfering effects of the various disturbancescan be properly weighted in deriving this composite characteristic.

What is claimed is:

1. A multiplex signaling system comprising two terminal stations, atransmission line interconnecting them and means for establishing amultiplicity of carrier wave channels for the transmission of signalsfrom one station to the other, means at said one station for applyingsignals each to a respective one of said channels and means at the otherof said stations for selectively receiving said signals, said systembeing subject to interference which has a net disturbing eifect onsignal transmission that is non-uniform over the frequency rangeoccupied by said channels so that the ratio of signal level to theeffective disturbance level tends to vary from channel to channel, andmeans for substantially equalizing the said ratio for all of thereceived signals comprising means at each of several points between saidterminal stations for shifting each of said signals to a successivelydifferent channel, each of said channels successively carrying severaldifferent signals.

2. In a signaling system, two terminal stations and means providing amultiplicity of carrier wave channels for the transmission of signalsfrom one of said stations to the other, means at one of said terminalstations for applying signals to respective ones of said channels, meansat the other of said terminal stations for selectively receiving signalstransmitted over said channels, and means at each of severalintermediate points in the transmission system for transferring each ofsaid signals to a frequency channel other than one previously occupiedby said signal.

3. In a system comprising a pair of terminal stations and severalintermediate stations all connected by a. transmission line, the methodof multiplex transmission which comprises applying to said line at oneof said terminal stations a multiplicity of bands of high-frequencywaves each bearing respective telephone signals, selectively receivingsaid bands of waves at each of said intermediate stations and at theother of said terminal stations, and at each of said intermediatestations reducing the said telephone signals to their originalfrequencies and reapplying each of them to said line in a respectivefrequency band other than one previously occupied by it in'itstransmission from said one terminal station.

4. The method of multiplex transmission of signals over a system havingseveral sections or links which comprises transmitting a multiplicity ofsignals in respective frequency bands over the first link of the system,changing the relative positions of said signals in the frequencyspectrum and transmitting them over the second link of the system, andsimilarly changing the relative frequency positions of said signals fortransmission over each succeeding link, the successive relativepositions of the said signals being such that each of said signalsoccupies as many different relative positions as there are links in thesystem.

5. In a multiplex carrier wave wire line transmission system, whereinterference from crosstalk, static, noise and the like is a function offrequency and greater in some carrier channels than in others, themethod of reducing the'efiect of such interference which comprisessystematically reassigning each signal to a diiferent frequency channelat several successive points along the line, the interference in thesuccessive channels to which each signal is successively assigned beingsuch that the cumulative interference effect is substantially the samefor all of said signals.

MYRON ALEXANDER WEAVER.

